1. Field of the Invention
The present invention relates to a scanning probe microscope for obtaining optical information relating to a surface of a sample by using a probe. The present invention relates, for example, to a scanning near-field optical microscope.
2. Description of the Related Art
The term "scanning probe microscope (SPM)" generically represents an apparatus for detecting an interactive effect caused between a probe and a surface of a sample when the probe is at a distance of 1 .mu.m or less from the surface, while the probe is scanned in the X and Y or X, Y and Z directions, and for mapping the interactive effect in a two-dimensional manner, The term SPM inclusively encompasses, for example, scanning tunneling microscopes (STMs), atomic force microscopes (AFMs), magnetic force microscopes (MFMs), and scanning near-field optical microscopes (SNOMs).
SNOMs have been developed specifically since the second half of the 1980s as optical microscopes capable of achieving a resolution that is higher than a diffractometric limit by detecting an evanescent wave, and such property of the evanescent wave is employed in the principle of measurement so that the evanescent wave is localized in a region dimensionally less (in thickness) than a wavelength, and never propagated in a free space.
U.S. Pat. No. 5,272,330 issued to Betzig et al. on Dec. 12, 1993, discloses an SNOM. In the SNOM, an evanescent field is generated in the vicinity of a fine aperture in a tip of a probe by introducing a light to the probe which is tapered in the leading end thereof. The evanescent field is brought into contact with a sample, light caused by the contact between the evanescent field and the sample is detected by means of an optical detector which is placed below the sample, and the intensity of transmitted light is mapped in a two-dimensional manner.
In the SNOM, a bar-shaped probe is employed as an optical fiber or glass bar or crystal probe with a tapered tip. An improved bar-shaped probe coated, except in a tip area thereof, with a metal film is now commercially available. The probe is produced by rotating a bar-shaped probe such an optical fiber or glass bar or crystal probe with a tapered tip, while a metal is deposited thereon from behind it at an angle. An apparatus using such a probe provides a lateral resolution higher than that of an apparatus using a probe without a metal coating.
On the other hand, the AFM is the most popular of SPMs as an apparatus for obtaining information of configuration of a sample surface. In the AFM, the information on configuration of a sample is obtained indirectly by detecting displacement of a cantilever which is caused in response to a force acting on a probe by means of an optical displacement sensor, for example, when the probe supported in a distal end of the cantilever is brought to the vicinity of the surface of the sample. One type of AFM is disclosed, for example, in Japanese Patent Publication No. 62-130302.
The technique for measuring configuration of a sample by detecting an interactive force between a sample and a probe tip in the AFM is also applied to other SPMs, and is used as means for constantly maintaining a distance between a sample and probe tip, that is, so-called regulation.
N. F. van Hulst et al. suggested a novel SNOM for detecting optical information of a sample, while configuration of the sample is measured according to the AFM measurement technique by using a cantilever of silicon nitride. This novel SNOM is disclosed in Appl. Phys. Lett., 62 (5), p. 461 (1993). In the apparatus, the sample is placed on an internal total reflection prism, a HeNe laser beam is applied from the side of the total reflection prism to the sample, the sample is excited, and an evanescent light field is generated in the vicinity of a surface of the sample. Then, a probe supported in a distal end of the cantilever is inserted to the evanescent light field, the evanescent light field, which comprises a localized wave, is converted to scattering light which comprises a propagating wave. A part of the light propagates through the inside of the probe of silicon nitride which is almost transparent to the HeNe laser beam and out of a back side of the cantilever. The light is collected by a lens located above the cantilever, and applied to a photomultiplier through a pinhole which is provided in a position conjugate with the probe tip in respect to the lens, and an SNOM signal is outputted from the photomultiplier.
While the SNOM signal is detected, the cantilever is measured for displacement thereof by an optical displacement detecting sensor, and a piezoelectric scanner, for example, is feedback-controlled so that the displacement is maintained to a predetermined value. Thus, in a scanning operation, an SNOM measurement is achieved according to scanning and SNOM signals, and an AFM measurement is achieved according to scanning and feedback control signals.
In the SNOM of either Betzig et al. or N. F. van Hulst et al., the probe tip is required to be optically transparent. In addition, to achieve a high lateral resolution for an SNOM image, the probe is desirably coated with metal. However, it is not easy to uniformly mass-produce such a metal-coated probe having an aperture in a tip thereof. In the SNOM expected for a super-resolution, a resolution higher than that achievable by the usual optical microscope is demanded, and the diameter of the aperture in the probe tip is required to be 0.1 .mu.m or less, preferably 0.05 .mu.m or less, in order to achieve such resolution. It is very difficult to produce an aperture of such size with a high reproducibility.
Furthermore, because an amount of light applied to the probe through the aperture is reduced in proportion to the second power of a diameter of the aperture, it is a problem of trade-off that an amount of light detected is reduced, if a diameter of the aperture is reduced for providing a higher lateral resolution of an SNOM image, and the S/N of a detection system is lower.
In the SNOM of N. F. van Hulst et al., since light transmitted through the probe of silicon nitride is detected, it is difficult to detect evanescent light in a range of short wavelengths. For example, a silicon nitride film is significantly reduced in spectral transmittance at a wavelength of 450 nm or less, even if the stoichiometry of silicon and nitrogen is 3:4. For a silicon nitride film for a cantilever, those with a higher content of silicon are generally employed in order to provide a silicon nitride film of low stress. In such silicon-rich silicon nitride film, reduction of the spectral transmittance in the range of short wavelengths is more significant. Therefore, the SNOM measurement using a cantilever of silicon-rich silicon nitride in the range of short wavelengths is almost unachievable, or the S/N is very poor, even if it is achieved.
A novel SNOM without an aperture in a probe tip is proposed. With the SNOM, because it is not required to provide an aperture in a probe tip, such difficulty in producing an aperture and problem of trade-off as described above are avoided. The novel SNOM is also referred to as a scattering mode SNOM, since scattering light propagating outside a probe is detected as SNOM information. Owing to the detecting method, a problem of difficulty in detecting evanescent light in the range of short wavelengths is also avoided.
In Japanese Laid-open Patent 6-137847, a scattering mode SNOM is disclosed. The SNOM converts evanescent light formed on a surface of a sample to propagating light by scattering it with a needle-like probe, detects the propagating or scattering light by using a condenser lens located in a side of the probe and a photoelectric detector, and provides optical information of the sample according to the detected signal.
In the SNOM, because the probe and a scanner for scanning the probe in the X, Y and Z directions are provided above the sample, an objective of an optical microscope cannot be placed above the sample. Thus, an optical microscopic observation must be conducted either in the oblique or lateral direction. In such optical microscopic observation, it is difficult to determine a position of the probe on the sample at a high accuracy. It is also difficult to achieve a variety of general optical microscopic observations, polarization microscopic observation and Nomarski differential interference microscopic observation, for example. Besides, the objective may not be of high magnification and high NA, and it is, therefore, difficult to observe the sample at the high resolution. This is significantly disadvantageous for carrying out an SNOM measurement while simultaneously referring to a result of optical microscopic observation.
At the 42th Seminar of Japan Applied Physics Related Associations (Prepared Papers No. 3, p. 916, March, 1995), Kawata et al. disclosed an apparatus capable of achieving STM and SNOM analyses using a metal probe for the STM. In the apparatus, propagating light is generated when evanescent light produced on a surface of sample is scattered by a tip of the metal probe. The propagating light, thus generated, is detected with a detector located at the side of the probe and sample, while a distance between the sample and the probe is controlled by the STM.
In the apparatus of Kawata et al., because the distance between the sample and probe is controlled by the STM, samples to be observed are limited to those having an electrical conductivity. Such limitation is very disadvantageous, since nonconductive samples are generally much common than conductive ones.